Common terminology involves naming such polymers by reference to the structure of the repeating unit (as is standard in polymer chemistry) with families being named according to the sequence of ether (symbolised by “E”) and ketone (symbolised by “K”) linkages in the repeat units. For example, polymers consisting essentially of the repeating unit: —Ar—O—Ar—C(═O)—Ar—C(═O)— would be referred to as “PEKK”.
Polyarylether ketones (or “PEKs) have a variety of useful properties, such as excellent electrical insulating and mechanical properties at high temperature, high strength, toughness and resistance to heat and chemicals. Such polymers may be amorphous or semi-crystalline. Both types usually exhibit high glass transition temperatures (Tg), while the semi-crystalline forms also exhibit high melting temperatures (Tm). Amongst these polymers, the PEK, PEKK, PEEK, PEEKK and PEKEKK families are of particular interest for use in preparing biomedical implants and implant materials due to their excellent mechanical properties, chemical inertness and resistance to stress cracking. The same materials are also useful in aerospace and many other wide-ranging industrial applications including the preparation of thermoplastic composites.
The production of amino-terminated PEEK is described in Corfield et al. (“Synthesis and Colorimetric curing study of Amino-terminated PEEK oligomers” J. Polymer Sci. (1992) 30:845-849). This polymer is produced via a nucleophilic process in which a meta-aminophenol end-capper is used to terminate the linear polymer chains. The purpose of producing this polymer was to study the behaviour of the system on curing. Specifically, it was found that thermal cross-linking produced a highly cross-linked and amorphous solid. There is no suggestion in Corfield that there would be any particular benefits to using amino-terminated PEEK or that the process could be extended to other PAEKs.
An improved process for the production of PEKK is described in WO 2011/004164 (Ketonex). This process is electrophilic (i.e. a Friedel-Crafts process) and uses a Lewis acid and a controlling agent such as benzoic acid. This dispersion process can yield particles of a PEKK product having a uniform shape and a controllable size distribution. WO '164 is silent as to the use of capping agents other than those having phenyl or chlorophenyl end-groups and provides no teaching towards incorporating functionalised end-caps into PEKK.
As noted above, PAEKs have wide-ranging uses due to their excellent mechanical properties, chemical inertness and resistance to stress cracking. However, some applications of the polymers would benefit from further functionality. There thus exists a need for functionalised PAEKs and methods for their preparation.
The present inventors have surprisingly found that amine-functionalised (e.g. amine-terminated) PAEKs can be produced using a process in which the reactive amine end-cap is protected during the reaction and subsequently de-protected during the final work-up. The invention thus conveniently enables the addition of amine groups when performing the polymerisation reaction, i.e., no additional steps are required on order to achieve amine functionalisation. The use of protecting groups is common in pharmaceutical organic chemistry, but is generally avoided in polymer chemistry and other industrial applications due to the additional cost and complexity it can add to a process.